The characteristics of single phase and multi-phase induction motors are generally well known. This is so, irrespective of whether the motor is of the standard squirrel cage type intended to operate at a fixed line frequency, e.g., 60 Hz, or whether the motor is intended to operate at an applied line frequency which varies. The latter situation results when a frequency controlled inverter is used to power the motor.
One known characteristic is that an induction motor which operates at a constant voltage and line frequency exhibits an output torque which, at some motor speeds experienced at startup and during acceleration, is undesirable for certain applications. That is, its output torque tends to be relatively low at standstill and increases gradually to a maximum torque value at an intermediate speed of about 75-80% of rated speed. This torque is sometimes called the breakdown or stall torque. That is, an imposed load which presents a torque requirement greater than the breakdown torque will cause the motor to stall. Induction motors powered from a constant voltage, constant frequency source are widely used. In fact, such installations are among the most common found anywhere.
The relatively low torque available at standstill means that for certain types of loads, e.g., those having higher torque requirements, the load acceleration time may be undesirably long. Even more undesirable is the fact that during acceleration, the motor current is well in excess of that which prevails at rated load and rated speed. Motor overheating and, possibly, physical damage can result.
Somewhat the same kind of difficulty attends the use of variable frequency controllers applied with common induction motors. Such controllers are often embodied as frequency inverters and include a variable frequency supply in which the ratio of voltage magnitude to frequency is maintained substantially constant over the entire frequency range.
This known technique maintains a substantially constant level of magnetic flux in the motor. Such an inverter-based drive system provides the ability to operate the motor at different speeds under load. An additional benefit is that such a drive system provides an increased output torque at reduced motor current for each starting energization of the motor.
For induction motors powered by a constant voltage, constant frequency source, the relatively low starting and acceleration torque characteristics are due in large part to the presence of leakage and magnetizing reactances. Similarly, the benefits obtainable from an inverter-based variable frequency drive system used with an induction motor tend to be limited by the same factors.
Control designers have recognized some of the foregoing disadvantages and have attempted to overcome them with additional control elements. An example of such an effort is shown in U.S. Pat. No. 4,063,135. This apparatus adds capacitors to the motor stator circuit, one of which is controllably switched in and out of the circuit.
The apparatus in U.S. Pat. No. 4,675,565 recognizes the desirability of using resonant circuits to improve motor performance while the apparatus of U.S. Pat. No. 4,808,868 uses what are termed quasi resonant circuits. Other designers have used non-linear control circuit elements (U.S. Pat. No. 2,040,763), chokes (U.K. Specification No. 617,704) or variable resistors (U.S Pat. No. 4,450,399) as motor control elements. The apparatus of U.S. Pat. No. 2,646,538 uses a saturable reactor and a capacitor to modify motor characteristics, both of these elements being of fixed value and electrically connected to the motor circuit.
While these earlier efforts have achieved a degree of success in improving the performance characteristics of induction motors, they have failed to appreciate the best manner in which the output torque and other characteristics of such a motor may be improved, irrespective of whether the motor is operated at a voltage having a variable or a fixed applied frequency.
More specifically, they have failed to recognize the way in which a capacitive reactance may be electromagnetically reflected into the motor circuit and how the value of this capacitive reactance may be controlled as a function of either one of two motor parameters. For an induction motor operated at a fixed line frequency, the parameter is speed as viewed substantially from standstill to rated speed. For a motor having a variable frequency applied thereto, the parameter is the applied frequency.
An improved motor controller which electromagnetically introduces a capacitive reactance into the motor circuit for neutralizing rotor leakage and magnetizing reactances and which varies this capacitive reactance in accordance with a motor parameter to provide improved output torque and, in certain instances, reduced motor current would be an important advance in the art.